Giardia duodenalis, an intestinal parasite of man and animals, is the most frequently identified pathogen in waterborne outbreaks of disease in the United States. Cysts of this parasite have been found in recent years in many of our surface waters and a number of waterborne outbreaks of giardiasis have been recorded. Because of the human health risk associated with the waterborne spread of Giardia, efforts have been directed to the development of procedures for the detection of cysts of the parasiste in water. A tentative procedure which has been reported to have a relatively low cyst recovery rate has been published as a starting point for further research. More sensitive methods are needed, however, to accurately assess potential human health risks in water due to this parasite. The main objective of the proposed research is to improve methods for the detection of Giardia cysts in water. Studies will be conducted: 1) to determine the best filter type for recovery of Giardia cysts from water, 2) to determine the most effective means for removing cysts from filters for subsequent detection, and 3) to identify the optimum conditons for transporting and storing filters prior to analysis. A modification of the tentative procedure will be used to detect Giardia cysts in the samples and to quantify the number of cysts recovered. A sufficient number of replicates of each experiment will be conducted to allow the data to be analyzed statistically. These studies will contribute to the development of reliable procedures for assaying water for potential health risks due to Giardia.